Exemplary embodiments of the present invention relate to an image sensing device, and more particularly, to a circuit for handling an image signal provided from a pixel.
An image sensing device such as a portable camera or a digital camera typically generates an image from a subject of the world. Examples of a sensing element in an image sensing device include a CMOS image sensor and a charge coupled device (CCD) image sensor. A CMOS image sensor or CCD image sensor has an array of pixels that respond to light for capturing images.
Various subjects in the natural world may have varying brightness and wavelengths of light. Each pixel of an image sensor corresponds to an electrical value depending on light from that subject. The electrical value is converted into an electrical voltage that can be processed in the image sensor. An image sensor includes an analog-to-digital (A/D) converter which converts analog voltages sensed by a pixel array into digital values and a memory for storing values during signal processing.
The pixel array is typically implemented as a matrix. Each pixel is arranged at the predetermined point of the matrix. Traditionally, a pixel includes four MOS transistors and a photodiode. The photodiode accumulates charges corresponding to incident light from the external. Three of the four MOS transistors serve as a switching element in the pixel and the other transistor serve as a source follower.
A required operating speed of an image sensing device has continuously increased. Hence, there are some limitations to increasing a signal processing speed of an image sensing device.